He was the subject of a documentary film titled Extraordinary People: The Boy with the Incredible Brain, first broadcast on the British television station Channel 4 on 23 May 2005.[11]

He met software engineer Neil Mitchell in 2000. They lived in Kent, England, where they had a quiet life at home with their cats, preparing meals from their garden.[12] He and Mitchell operated the online e-learning company Optimnem, where they created and published language courses.

Tammet now lives in Paris (France)[13] with his husband Jérôme Tabet, a photographer whom he met while promoting his autobiography.

Tammet is a graduate of the Open University with a Bachelor of Arts degree (First class honours) in the Humanities.[14]

In 2002, Tammet launched the website, Optimnem.[15] The site offers language courses (currently French and Spanish) and has been an approved member of the UK National Grid for Learning since 2006.[9]

Born on a Blue Day received international media attention and critical praise. Booklist magazine contributing reviewer Ray Olson stated that Tammet's autobiography was "as fascinating as Benjamin Franklin's and John Stuart Mill's" and that Tammet wrote "some of the clearest prose this side of Hemingway". Kirkus Reviews stated that the book "transcends the disability memoir genre".

His second book, Embracing the Wide Sky, was published in 2009.[17][18] Professor Allan Snyder, director of the University of SydneyCentre for the Mind, called the work 'an extraordinary and monumental achievement'.[19] Tammet argues that savant abilities are not "supernatural" but are "an outgrowth" of "natural, instinctive ways of thinking about numbers and words". He suggests that the brains of savants can, to some extent, be retrained, and that normal brains could be taught to develop some savant abilities.[19]

Thinking in Numbers, a collection of essays, was first published in 2012 and serialised as BBC Radio 4's Book of the Week in the United Kingdom.[20]

His translation into French of a selection of poetry by Les Murray was published by L'Iconoclaste in France in 2014.[21]

Tammet's first novel, Mishenka, came out in France and Quebec in 2016.[22]

Every Word is a Bird We Teach to Sing, a collection of essays on language, was published in the UK, US, and France in 2017.[23] In a review of the book for the Wall Street Journal, Brad Leithauser noted that "in terms of literary genres, something new and enthralling is going on inside his books" and that the author showed "a grasp of language and a sweep of vocabulary that any poet would envy".[24]

After the World Memory Championships, Tammet participated in a group study, later published in the New Year 2003 edition of Nature Neuroscience.[25] The researchers investigated the reasons for the memory champions' superior performance. They reported that they used "strategies for encoding information with the sole purpose of making it more memorable", and concluded that superior memory was not driven by exceptional intellectual ability or differences in brain structure.[26]

In another study, Baron-Cohen and others at the Autism Research Centre tested Tammet's abilities in around 2005.[27] Tammet was found to have synaesthesia according to the "Test of Genuineness-Revised" which tests the subjects' consistency in reporting descriptions of their synaesthesia. He performed well on tests of short term memory (with a digit-span of 11.5, where 6.5 is typical). Conversely, test results showed his memory for faces scored at the level expected of a 6- to 8-year-old child in this task. The authors of the study speculated that his savant memory could be a result of synaesthesia combined with Asperger syndrome, or it could be the result of mnemonic strategies.

Baron-Cohen, Bor and Billington investigated whether Tammet's synaesthesia and Asperger syndrome explained his savant memory abilities in a further study published in Neurocase in 2008. They concluded that his abilities might be explained by hyperactivity in one brain region (the left prefrontal cortex), which results from his Asperger syndrome and synaesthesia.[28] On the Navon task, relative to non-autistic controls, Tammet was found to be faster at finding a target at the local level and to be less distracted by interference from the global level.[28] In an fMRI scan, "Tammet did not activate extra-striate regions of the brain normally associated with synaesthesia, suggesting that he has an unusual and more abstract and conceptual form of synaesthesia".[28] Published in Cerebral Cortex (2011), an fMRI study led by Professor Jean-Michel Hupé at the University of Toulouse (France) observed no activation of colour areas in ten synaesthetes.[29] Hupé suggests that synaesthetic colour experience lies not in the brain's colour system, but instead results from "a complex construction of meaning in the brain, involving not only perception, but language, memory and emotion".[30]

In his book Moonwalking with Einstein (2011), science journalist and former US Memory Champion Joshua Foer speculates that study of conventional mnemonic approaches has played a role in Tammet's feats of memory. While accepting that Tammet meets the standard definition of a prodigious savant, Foer suggests that his abilities may simply reflect intensive training using standard memory techniques, rather than any abnormal psychology or neurology per se. In a review of his book for The New York Times, psychologist Alexandra Horowitz described Foer's speculation as among the book's few "missteps", questioning whether it would matter if Tammet had used such strategies or not.[31]

Tammet has been studied repeatedly[10] by researchers in Britain and the United States, and has been the subject of several peer-reviewed scientific papers.[17] Professor Allan Snyder at the Australian National University has said of him: "Savants can't usually tell us how they do what they do. It just comes to them. Daniel can describe what he sees in his head. That's why he's exciting. He could be the 'Rosetta Stone'."[8]

In his mind, Tammet says, each positive integer up to 10,000 has its own unique shape, colour, texture and feel. He has described his visual image of 289 as particularly ugly, 333 as particularly attractive, and pi, though not an integer, as beautiful. The number 6 apparently has no distinct image yet what he describes as an almost small nothingness, opposite to the number 9 which he calls large, towering, and quite intimidating. He also describes the number 117 as "a handsome number. It's tall, it's a lanky number, a little bit wobbly".[9][32] In his memoir, he describes experiencing a synaesthetic and emotional response for numbers and words.[9]

He is a polyglot. In Born on a Blue Day, he writes that he knows ten languages: English, Finnish, French, German, Lithuanian, Esperanto, Spanish, Romanian, Icelandic, and Welsh.[9] In Embracing the Wide Sky, he writes that he learned conversational Icelandic in a week and then appeared on an interview on Kastljós on RÚV speaking the language.[17][40]

Mänti is a constructed language that Tammet published in 2006.[47] The word 'Mänti' comes from the Finnish word for 'pine tree' (mänty). Mänti uses vocabulary and grammar from the Finnic languages. Some sample words include: